The influence of carboxylic acids in sulphuric acid anodising solutions on
the corrosion and SCC behaviour of Al-1050

Abstract

The aim of the present work is the study of corrosion and stress corrosion
cracking behaviour of 1050 Al-Alloy anodised in a 3M H2SO4
anodising bath in the presence of various carboxylic acids, saturated or not,
with different numbers of carboxylic, methyl or hydroxyl groups. The
investigation was carried out by SCC tests and electrochemical measurements.
The SCC behaviour of anodised 1050 Al-Alloy was found to vary with anodising
conditions, stress level and the differences in the structure and the carbonic
chain of the additives. Anodic coatings prepared in 3M H2SO4
without any additives, did not protect the bare alloy. Addition to the
anodising solution of any of the carboxylic acids used resulted in protection
of the alloy, with better protective properties in the case of malonic acid
and a high stress level. For the interpretation of the results, SEM
micrographs and IR spectra of the surfaces of the specimens were obtained.
Properties of the anodic coatings such as thickness, packing density, coating
ratio and roughness were also studied. The anodic coatings formed in a
electrolytic bath with additives present were found to be less porous, more
compact and rougher, having better anticorrosive and mechanical properties. An
explanation of the mechanism of the effect of carboxylic acids on oxidation
and SCC behaviour of Al-Alloys is the absorption of these compounds on the
metal surface. Corrosion and stress corrosion cracking behaviour is better in
the case of saturated bicarboxylic acids. The
presence of a methyl group improves the protective properties of the oxide but
an increase in their number has an opposite effect.

Introduction

The SCC behaviour of Al-alloys of the 1XXX series is rarely studied because
the high strength Al-alloys, eg. of the 7XXX series, are more important for
most industrial applications. Neverthless, Al-alloys of the 1XXX series are
widely used in many commercial applications and therefore studies of these
alloys are useful for estimating the effects of the various constituents of
high strength alloys on their corrosion behaviour.

Studies on behaviour of pure aluminium [1] in saline environments and in
stress corrosion conditions, conclude that as with other Al-Alloys [2,3], the
cracking is due to an atomic hydrogen absorption that causes crack growth and
increases dislocation activity. Also, depending on the applied potential,
different processes become prominent and the SCC at OCP is caused by pit
formation and crack initiation of pits [4].

Some Al-alloys used in practice are anodically oxidized, for protection
against corrosion and SCC. It has been shown that during anodising, a porous
Al2O3 film with a cellular structure up to a thickness
of 36 mm is formed, with cells oriented parallel to
the direction of gravity this resulting from the rapid evolution of oxygen in
the opposite direction. It was found that the protective properties of oxides
change with thickness and structure, being better against corrosion than
unanodised aluminium but worse against mechanical stress [5,6,7].

Many studies report [8-14] the effect of the addition of various organic
compounds on the corrosion behaviour of metals, mainly in the corrosive
environment, but these may also, in the case of aluminium, be in the anodising
bath during the electrolytic preparation of the oxides. The explanation given
for this effect is the absorption of compounds on the metal surface or the
formation of some complexes on it.

In earlier work [15-17] the effect on corrosion and stress corrosion
cracking behaviour of anodised 1050 Al-Alloy of the addition in an anodising
bath of 3M or 4M H2SO4 of some inorganic compounds [15],
bicarboxylic acids in 3M H2SO4 [16] or carboxylic acids
in 4M H2SO4 [17] was examined.

In order to further investigate this effect, aim of the present work is the
study of corrosion and stress corrosion cracking behaviour of 1050 Al-Alloy
anodised in a 3M H2SO4 anodising bath in the presence of
various carboxylic acids, saturated or not, with different numbers of
carboxylic, methyl or hydroxyl groups. The investigation was carried out by
SCC tests and electrochemical measurements.

Experimental methods

The anodic coatings on Al-Alloy surface were prepared electrolytically in a
bath of 3 M H2SO4 with an addition of 0.015 M of the
following carboxylic acids: acetic, malonic, succinic, phthalic, glutaric,
adipic, fumaric, citric.

The anodising current density was 600 A/m2, at 25�C and the
coatings thickness was estimated (6) to 10 mm. The
real thickness of the coatings was confirmed by direct microscopic examination
of the cross sections of the anodised specimens.

SCC tests

The specimens were cut from a plate of 0.30 mm thickness and have a middle
section of reduced width where the cross section was 1 mm2. The
total exposed area of the specimens was 1.28 cm2 and the rest was
masked with insulating varnish to eliminate any parasitic interactions during
testing (Fig. 1). The specimens were stressed directly by loading at a
definite stress level and the time to failure (TTF) was derived. Experiments
at three different stress levels (4.17, 6.25, 8.33 kg/mm2) were
carried out. During the test, the corrosive environment was renewed at a
constant rate of 70 ml/h. A galvanostatically controlled anodic current of 5
A/m2 was impressed during the
test. Six specimens were tested under each set of conditions.

Figure 1. a) Shape and dimensions of specimens (AA'-BB': anodizing area
for the case of anodized specimens). b) Specimens position during SCC tests.
(Click the figure for an enlarged view)

To ensure that the SCC mechanism was the predominant factor for the failure
of the specimens during the SCC tests, the following factors were included in
the design of this experiment. The applied stresses σ
were in the region where SCC is defined (threshold stress = 1.75 kg/mm2
< s = 4.17, 6.25, 8.33 kg/mm2 < yield point = 9.41 kg/mm2).
The impressed anodic current densities to accelerate the SCC phenomenon were
low: 5 A/m2. The present authors took into account the results of a
study of the influence of the pure electrochemical dissolution during SCC
tests of aluminium alloys in saline water [18] where it was found that
"when testing under anodic SCC conditions and the tests are carried out
in the region where a true SCC mechanism is valid, anodic dissolution is not
the predominant factor" [18]. These results were obtained from TTF
measurements of specimens under the same experimental conditions as in this
work. The specimens were at first pre-exposed under the same conditions of
applied anodic current without load, and then loaded to fracture.

Polarization tests

The specimens were also cut from a plate of the same thickness (0.30 mm),
the dimensions were 1 cm x 5 cm and the total exposed area was 2 cm2
(the rest was also masked with insulating varnish).

The cyclic anodic potentiodynamic polarization measurements were carried
out with standard methods [19-21], without any agitation or renewal of the
solution, at a slow scan rate of 0.6 V/h. Before starting the potential scan,
the specimens were immersed in the test solution for 1 h to reach a steady
state of equilibrium (open circuit corrosion potential, Ecor). A
potentiostat-galvanostat (Bank PGS-81) with a scan generator (Bank VSG-72) and
a X-Y recorder, platinum counter electrode and saturated calomel reference
electrode (SCE) were used.

Surface examination

Scanning electron microscopy (SEM) was used to study the surface of the
specimen. The SEM experiments were carried out with a JEOL, JSM-840 A Scanning
Microscope, connected with a Energy Dispenser Spectrometer - EDS - (LINK, AN
10/55S).

Packing Density

For the calculation of the packing density, the weight of the anodic
coating, its thickness and the oxidised area of the specimen were measured.
The packing density was calculated as the ratio of weight to volume of each
anodic coating.

Coating Ratio

The coating ratio, i.e., the coulombic efficiency for the formation of
porous oxide films on Al, was calculated by the ratio of the weight of oxide
formed to the weight of aluminium consumed [22].

Roughness

The roughness of surface of the anodic coatings was measured with a
perthometer (PERTHEN C 5D with a tracer drive unit PVK) and the roughness
factor that was used in our experiments was the mean arithmetic value of all
the distances of the roughness diagram from its central line.

Infrared Reflection spectrum (IR)

The Infrared Reflection spectrum method was used for the determination of
the qualitative composition of the anodic coatings. Spectra were taken with an
infrared spectrometer with Fourier transformation (FFT-IR, Bruker IFS 113,
lamp Globag I, beamsplitter KBr, detector DTGS with window KBr and 5000-400cm-1
measurement area).

Results and Discussion

In Figure 2, SCC results (increase % of TTF of anodised against bare
specimens) of samples anodised in 3M H2SO4 and with an
addition of 0.015 M of various carboxylic acids are shown. The SCC behaviour
was found to vary with anodising conditions and stress level. Anodic coating
prepared in 3M H2SO4 without any additives did not
protect the bare alloy, decreasing TTF. Addition to the anodising solution of
any of the carboxylic acids used resulted in protection of the bare alloy. The
increase of TTF was greater for the saturated bicarboxylic acids and shows a
maximum value in the case of malonic acid, the protective properties being
better as stress level increases.

In Figure 3 the potentiodynamic polarization curves of bare or anodised
specimens in 3M H2SO4 and with an addition of 0.015 M of
malonic or citric acids are shown. From these results it follows that in all
cases the Al-Alloy suffers localized corrosion. This is indicated by Epit
having a more positive value than Ecor and also from the presence
of a hysteresis loop between the forward and reverse scans [23,24]. In all
cases the Al-Alloy is not passivated, as passive regions do not appear for
potential values more positive than Epit and reverse currents are
always higher than forward currents. It is also observed that the addition of
carboxylic acids did not significantly influence Ecor but shifted Epit
in the noble direction (-610 mV for malonic acid) relative to the bare alloy
(-680 mV) and decreased the anodic current indicating less susceptibility to
localized corrosion in the free corrosion potential regions. In the absence of
any carboxylic acid, Epit shifted in the opposite direction (-720
mV for 3 M H2SO4).

The infrared reflection spectra (IR) are shown in Figure 4. The peaks
observed in the areas 3500-3200cm-1 and 1625 cm-1
indicate the presence in the oxides of hydroxyl groups and absorbed water. The
peak at 1460cm-1, observed in all coated specimens, corresponds
with the C-O bond. The presence of this peak in the case of the oxide prepared
in 3M H2SO4 without any additives it is due to the
chemical absorption of CO2 from the environment onto the oxide. The
peak at 1200cm-1 corresponds with the S-O bond of the sulphide ions
that are absorbed in the oxide during anodization and the peaks in the 1080cm-1,
970cm-1, 940cm-1 and 570cm-1 are due to
vibrations of the Al-O bond. The detection of such bonds is in accordance with
the results of other works [25], where the spectroscopic study of the
aluminium oxide prepared in H2SO4 showed a triple
structure for the oxide, composed mainly from coatings of aluminium hydroxyl,
aluminium hydroxide and hydrated aluminium oxide. The presence of carbonate
and sulphate ions was also detected (Fig. 5).

Both corrosion and especially stress resistance of the coating are better
when prepared in the presence of malonic acid as shown from the SEM
micrographs of SCC tested specimens, coated in 3 M H2SO4
+ 0.015 M malonic acid baths (Fig. 6).

The measurements of the physical properties of the anodic coatings shown in
Table 1 indicate that the addition of carboxylic acids during anodising
decreases thickness and increases packing density of the coatings resulting in
the formation of a less porous oxide layer. The coating ratio decreases in the
presence of additives, while roughness increases. These results can be
attributed to the lower presence of SO3 in the oxide film (6.48% in
presence of malonic acid and 7.36% without the additive), due to lower
incorporation of anions into the oxide structure during anodising in presence
of the additives and also to the possible dissolution of the outer surface of
the oxide during anodising [16]. An explanation of the mechanism of the effect
of carboxylic acids on oxidation, corrosion and SCC behaviour of Al-Alloys is
the absorption of these compounds on the metal surface. The different
behaviour of the coatings prepared in the presence of various carboxylic acids
must be explained by the differences in the structure and the hydrocarbon
chain of these compounds. Corrosion and stress corrosion cracking behaviour is
better in the case of bicarboxylic acids and worse in the cases of one (acidic
acid) or three (citric acid) carboxylic groups present. It is also better in
the case of saturated acids than in the presence of double bonds (maleic,
fumaric, phthalic acids). The presence of methyl groups improves the
protective properties of the oxide, which are worse in the absence of these
(oxalic acid) or in the presence of hydroxyl groups (tartaric acid). Increase
of the number of the methyl groups from one to four (malonic, succinic,
glutaric, adipic acids) makes the protective properties of the oxide worse.

Conclusions

The SCC behaviour of anodised 1050 Al-Alloy was found to vary with
anodising conditions, stress level and the differences in the structure and
the carbonic chain of the additives. Anodic coatings prepared in 3M H2SO4
without any additives, did not protect the bare alloy. Addition to the
anodising solution of any of the carboxylic acids used resulted in protection
of the alloy, with better protective properties in the case of malonic acid
and a high stress level.

The addition of carboxylic acids during anodising decreases thickness and
coating ratio and increases packing density and roughness of the coatings,
resulting in the formation of a less porous oxide layer.

An explanation of the mechanism of the effect of carboxylic acids on
oxidation and SCC behaviour of Al-Alloys is the absorption of these compounds
on the metal surface. Corrosion and stress corrosion cracking behaviour is
better in the case of saturated bicarboxylic acids.

The presence of a methyl group improves the protective properties of the
oxide but an increase in their number has an opposite effect.